1. Field of the Invention
This invention relates to a tone detection arrangement of a type in which the tones of an electrical multi-tone signal comprising a plurality of sequential tone bursts can be successively detected: the arrangement provides recognition of the signal, as received by it, only if it is able to detect all of the tones of the signal.
A tone detection arrangement of the above type has application, for example, in the mobile stations of a mobile radio system for detecting multi-tone sequential call signals which are used to address uniquely the mobile stations. Thus, for instance, in a mobile radio system consisting of a fixed control station and a plurality of vehicle-mounted mobile stations, each mobile station is allocated a unique multi-digit number, and a speech or data message to be transmitted to a particular mobile station by the control station is preceded by a multi-tone sequential call signal the tones of which correspond to the digits of the multi-digit number allocated to that mobile station. The tone detection arrangement in each mobile station makes the recognition when the station is called, for instance by lighting a call lamp. This avoids the need for driver-radio operators in the mobile stations having to listen for their individual call signs, perhaps at the expense of attention to driving.
2. Description of the Prior Art
A presently known tone detection arrangement of the above type comprises a tuned circuit with a tapped coil. Respective taps on the coil tune the tuned circuit for resonance at different tone frequencies, a control unit being included in the arrangement for controlling the switching to a required tap. In operation, the arrangement is switched initially to the tap for which the tuned circuit is tuned to the frequency of the first tone of a multi-tone sequential call signal which the tone detection arrangement is to recognise. When in response to a received multi-tone sequential call signal the voltage across the tuned circuit exceeds a threshold voltage, signifying detection of the first tone, the control unit switches to the tap for which the tuned circuit is tuned to the frequency of the second tone of the multi-tone sequential call signal. If the voltage across the tuned circuit now exceeds the threshold voltage again, signifying detection of the second tone, switching to the next tap is effected, and so on. If all the tones of the multi-tone sequential call signal are detected, the tone detection arrangement provides recognition of the signal. Timing circuits in the arrangement ensure separation between the detection of the successive tones of the received multi-tone sequential call signal.
In an alternative form of such presently known tone detection arrangement, the tuned circuit is tuned for resonance at different tone frequencies by the control unit switching to different capacitors instead of to different coil taps.
The cost of such an analogue tone detection arrangement can be high, due to the number of discrete components required for its implementation and hence the number of soldered interconnections needed. This latter requirement may, in turn, contribute to unreliability. Furthermore, such an analogue tone detection arrangement can suffer from ageing, which requires it to be re-tuned periodically.